12 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> show index from tb_user; +---+---+---+---+---+---+---+---+---+---+---+---+---+---+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | ...
在SQL查询中一个比较常见查询应当是COUNT操作。对于带WHERE子句的查询没太多可说的,有索引就用索引,没有索引——嘿嘿,累坏机器啊!!!而不带WHERE子句简单COUNT查询的,比如count(*)、count(1)、count(col)之间又有什么区别呢? 下面我们就通过一组实验来验证一下,这三者之间到底有什么区别?
从执行计划来看,count(1)和count(*)的效果是一样的。但是在表做过分析之后,count(1)会比count(*)的用时少些(1w以内数据量),不过差不了多少。 如果count(1)是聚索引,id,那肯定是count(1)快。但是差的很小的。因为count(),自动会优化指定到那一个字段。所以没必要去count(1),用count(),sql会帮你完成...
数据库是我们必须掌握的知识，经常操作数据库不可避免，but，在写 SQL 语句的时候，难免遇到各种问题。
So it's no surprise that SQL has its own function to help. Whether you're identifying duplicates, calculating group totals, or filtering data, the COUNT() function is here to help. In this article, I'll show you the many ways in which COUNT() is useful, from its basic syntax to ...
1.What does COUNT() with DISTINCT do in SQL? COUNT() with DISTINCT counts the number of unique values in a specified column or expression, excluding duplicates. 2.Why would we use SQL COUNT(DISTINCT ...) instead of just SQL COUNT()?
,->('e','');QueryOK,8rowsaffected(0.01sec)Records:8Duplicates:0Warnings:0mysql>select*fromyy...
Introduction to SQL COUNT function TheCOUNT()function returns the number of rows in a group. The first form of theCOUNT()function is as follows: TheCOUNT(*)function returns a number of rows in a specified table or view that includes the number of duplicates andNULLvalues. ...
Records:10000000Duplicates:0Warnings:0 2、 查询一张表数据量的方法 查询一张表的数据量有如下几种: 查询大致数据量,可以查统计信息,2.1中会介绍具体方法 精确查找数据量,则可以通过count(主键字段),count(*), count(1) [这里的1可以替换为任意常量] ...
Yes, COUNT() can be used with the DISTINCT keyword to count only unique values in a column, excluding duplicates. 3.How does COUNT() handle NULL values? COUNT(*) includes all rows in its count, even those with NULL values in any column. In contrast, COUNT(column_name) counts only non...